A plant for production of synthetic diesel and other synthetic hydrocarbons consists of three main parts. In the first main unit, synthesis gas (a mixture of hydrogen and carbon oxides) is produced from the feedstock which is usually natural gas or a similar light hydrocarbon feedstock. In the second main unit, the actual hydrocarbon synthesis takes place usually by the Fischer-Tropsch synthesis. In the final part often known as the Product Workup unit (PWU) the raw products are refined and/or separated to give the desired end products. The present invention relates to an improved method for production of synthesis gas.
Today, one of the most cost effective and efficient methods for production of synthesis gas is by Autothermal Reforming (ATR). In ATR the light hydrocarbon feedstock with addition of steam reacts with a sub-stoichiometric amount of oxygen to produce synthesis gas. An ATR reactor consists of a burner, a combustion chamber and a catalyst bed in a refractory lined pressure shell.
For the Fischer-Tropsch synthesis to be as effective as possible, a specific synthesis gas composition is often desired. In many cases the desired synthesis gas composition is given by the ratio of the content of hydrogen to the content of carbon monoxide. The desired ratio is often approximately 2.0. With most operating conditions ATR is not able to produce this ratio. Instead a carbon dioxide containing stream must be recirculated to a location upstream the ATR reactor. This recirculation stream is often a tail gas, which is essentially a by-product from the Fischer-Tropsch synthesis unit and/or the Product Work-up unit. The main components in the tail gas are carbon monoxide, carbon dioxide, hydrogen and various light hydrocarbons such as methane, ethane, propane, ethylene and propylene.
Often, as described in the art (e.g. U.S. Pat. No. 6,375,916), an adiabatic prereformer is added upstream the autothermal reformer. In the pre-reformer the following reactions take place:CnHm+nH2OnCO+½(m+2n)H2(>=2)  (1)3H2+COCH4+H2O  (2)CO+H2OH2+CO2  (3)
At most conditions higher hydrocarbons (hydrocarbons with more than 1 carbon atom) are completely removed. The last two reactions (2) and (3) are close to thermodynamic equilibrium at the exit temperature of the adiabatic prereformer. Typically, the catalyst in the adiabatic prereformer is nickel on a ceramic carrier.
It is described in U.S. patent application Ser. No. 20010051662 by Arcuri et al. that mixing of tail gas and a hydrocarbon feedstock and feeding the resultant mixture to an adiabatic pre-reformer is advantageous for production of synthesis gas. However, according to the present invention, recirculation of the tail gas to the feed to the adiabatic prereformer is disadvantageous because the risk of carbon formation will be higher in the prereformer. This means that the process must be operated at a higher steam to carbon ratio (ratio of steam to carbon in hydrocarbons) to avoid carbon formation. It is generally recognised that operation at a low steam-to-carbon ratio is beneficial to the economics in a Fischer-Tropsch plant.
Steam reforming involves the risk of detrimental carbon formation on the catalyst. Carbon may deposit on the catalyst either from methane, carbon monoxide, higher paraffinic hydrocarbons, or other components such as olefins.
For methane the carbon forming reaction may be expressed by:CH4C+2H2  (4)
The composition assuming chemical equilibrium of the steam reforming and shift reactions (1–3) is calculated based on the feed stream composition and the temperature and pressure. This should in principle be done at each position in the reactor. However, experience shows that the risk of carbon formation from methane according to reaction (4) increases with temperature. Based on the calculated equilibrium composition, the reaction quotient for reaction (4) is calculated. The reaction quotient, Qc, is the ratio of the square of the partial pressure of hydrogen to the partial pressure of methane (P2H2/PCH4). If the ratio is higher than the equilibrium constant for reaction (4) at the same temperature, carbon is not predicted to form.
One method for reducing the required amount of steam without carbon formation is to use noble metal catalysts (Rostrup-Nielsen et al., J. of Catalysis 144, pages 38–49, 1993). However, the cost of noble metals as compared to nickel is very high and it is desirable to minimise the amount of catalyst.
Synthesis gas production may account for more than 50% of the total capital cost in a Fischer-Tropsch plant. For a plant based on ATR a large fraction of the cost of the synthesis gas production unit arises from the air separation unit needed to produce oxygen. Hence, there is a considerable interest in methods for reducing the oxygen consumption per unit of synthesis gas produced.